Here is how it goes:

Have the person write down any three digits number with

Reverse the number you wrote in step #1.

Subtract the number obtained in step #2 from the number you wrote in step #1

Reverse the number obtained in step #3

Add the numbers found in step #3 and step #4

Example #1:

Have the person write down any three digits number with decreasing digits: 752

Reverse the number you wrote in step #1: 257

Subtract the number obtained in step #2 from the number you wrote in step #1: 752 - 257 = 495

Reverse the number obtained in step #3: 594

Add the numbers found in step #3 and step #4: 495 + 594 = 1089

Example #2:

Have the person write down any three digits number with decreasing digits: 983

Reverse the number you wrote in step #1: 389

Subtract the number obtained in step #2 from the number you wrote in step #1: 983 - 389 = 594

Reverse the number obtained in step #3: 495

Add the numbers found in step #3 and step #4: 594 + 495 = 1089

Example #3:

Have the person write down any three digits number with decreasing digits: 210

Reverse the number you wrote in step #1: 012

Subtract the number obtained in step #2 from the number you wrote in step #1: 210 - 012 = 198

Reverse the number obtained in step #3: 891

Add the numbers found in step #3 and step #4: 198 + 891 = 1089

This number trick with 1089 works with any 3 digits number as long as you choose a number with decreasing digits in step #1

So, it does not matter what number the person choose in step #1, have the person do the math and pretend you are not looking. When you call the answer as 1089, the person will be shocked! Cool!